1. Field of the Invention
The present invention relates to hybrid electric vehicles, that is, vehicles in which both an internal combustion engine and at least one motor are provided to supply driving torque, a battery is provided to supply electrical energy to the motor, the motor is operable as a generator to receive torque to recharge the battery. More particularly, the present invention relates to a method and system for enhanced battery control that improves the performance of a hybrid electric vehicle.
2. Description of the Background Art
Hybrid electric vehicles have a battery and at least one motor that are operable as a generator. The battery operates in various modes, including discharge mode and recharge mode. In discharge mode, the battery is discharged to power a motor. In recharge mode, the motor is operable as a generator and receives torque from an internal combustion engine or road wheels to recharge the battery. Various measures of battery control have been proposed to improve performance of hybrid electric vehicles. One example of such measures is found in JP-A 11-164402 published Jun. 18, 1999. According to the conventional implementation, a controller predicts what will be requested on a battery in future based on operating condition of the vehicle, and raises or lowers SOC of the battery to meet various requests predicted.
While it can properly manage a battery of a hybrid electric vehicle to some extent, the conventional implementation does not have sufficient capability of being responsive to variations of electrical power during operation in discharge and recharge modes. According to the conventional implementation, a predetermined margin is set to fix a portion of capability of a battery usable for operation in discharge and recharge modes. If the margin is set to keep variations in SOC with an appropriate range, the conventional implementation has limited capability of supplying electrical power great enough to drive a motor when a great driving force is requested over a short period of time. Further, it has limited capability of receiving a great electrical power to raise SOC to a satisfactorily high level during operation in recharge mode. If the margin is set to a smaller value to permit a sufficiently great magnitude of electrical current during operation in discharge and recharge modes to meet the above-mentioned requests, it is difficult to keep variations in SOC within the appropriate range due to an excess magnitude of electrical current discharged from or recharged to the battery over the overall operating conditions of the hybrid electric vehicle.
It is thus seen to be desirable to enhance battery control to improve the performance of a hybrid electric vehicle.